I've hung onto John Barsness' "Bob Revisited" article from the Feb 2003 HANDLOADER for years now. This guy is an extremely experienced hunter / shooter / writer who has owned more than a dozen Bobs and he and his wife both use them a lot.
He had a lot to say about both the Ack Imp version and with the "short vs-long action" debate. His conclusions were interesting and well thought out.
John used the Ack Imp for a few years and dropped it. His honest gains were about 100 fps with 115 - 120 grain bullets, above what his regular +P handloads did. This is because of a rule of ballistics which states that for every gain in powder capacity, corresponding velocity is gained, at one-fourth the amount. Since the Ack Imp gains about 12% powder capacity, velocity gain is around 3%, or about 90 fps assuming an average 3000 fps. In his opinion, not worth going to the trouble of fireforming. That's not to say it's not a great cartridge, but the performance gain isn't as great as some folks believe.
Older rifles were often housed in short actions, and the throats were cut long, which can degrade accuracy. In modern rifles, this isn't a concern. Throats are generally cut shorter and the Roberts shoots fine in them. Those very few who think the Roberts gains any real power by loading it to 3 inches instead of 2.8 inches are right. It will gain about 25 to 30 fps, because the .2 inches gives about a 4% gain in powder capacity, or a 1% velocity gain. So action length isn't really a consideration for this cartridge either. Short and long will both work.
As to the .25-06 being a better cartridge or not? (This is my opinion, not John Barsness'). It is a couple hundred fps faster than the Roberts, and the difference is even more noticeable with factory ammo. I honestly believe it has a noticeable advantage in killing power at 300+ yards vs the Roberts. But I bought my Bob because I wanted an effective-to-300-yards deer rifle which would be low in recoil, and I got one. Though they are both mild kickers, I find a HUGE difference in the percieved recoil levels of the two cartridges. So I would opine that no, the .25-06 is not "better" than Bob, it's just more powerful. Then again, I have never shot a deer past 175 yards. Ever. If I thought realistically that shots over 300 yards were likely for me, I would probably have bought a .25-06 before a Bob.
If there are any other Barsness fans here, and I know there are, he is a member of
www.24hrcampfire.com using the name Mule Deer, and he uses his experience to answer tons of questions from members. He was kind enough to answer some questions I had concerning handloading, and he knows his stuff.